Method for coloring cellulosic materials using cationic pigment dispersion

ABSTRACT

A method of coloring a cellulosic material which includes a) dispersing pulped cellulosic material into water; and b) coloring the pulped cellulosic material by adding a cationic dispersion to the water, where the dispersion includes: (i) at least one pigment; (ii) water; and (iii) at least one quaternary salt of a styrene maleimide imide resin in an amount effective to disperse the pigment. The cationic dispersion may be prepared by (i) mixing, at 500 to 10,000 rpm, at least one pigment; water; and either (a) at least one a quaternary salt of a styrene maleimide imide resin or (b) at least one styrene maleimide imide resin in combination with at least one weak acid, thereby forming a dispersion premix; (ii) milling the dispersion premix in a mixer filled with ceramic, metal or glass beads for a period of time sufficient to reduce pigment agglomerates to primary particles, thereby forming a nonstandardized dispersion; and (iii) standardizing the dispersion against a color standard by adding water. The resulting cationic dispersion can be used to color cellulosic materials such as cotton and paper.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of coloring cellulosic materials suchas paper pulp and cotton. More particularly, this invention relates to amethod of coloring cellulosic materials using a cationic dispersionwhich contains at least one pigment, water, and at least one dispersingagent comprising a quaternary salt of a styrene maleimide imide resin.

2. Description of the Prior Art

Papermaking is a well-known process in which a cellulosic material,typically obtained from wood, is mechanically or chemically pulped,dispersed in water, formed into a planar sheet, dried and wound onto aroll for later use. The paper may be sized to modify its surfacecharacteristics, particularly water penetration, which is important forwriting and printing grades of paper. Additives such as fillers andoptical brighteners may be added to the pulp prior to sheet formation.Colorants such as dyes or pigments may also be added during thepapermaking process, either by coloring the paper pulp, or applying thecolorant to the paper surface by dip coating, spraying or pad printing.Pulp coloration is the most widely used type of paper coloration.

“Substantivity” is the ability of a dye or pigment to be adsorbed bycellulose fibers from an aqueous medium.

“Affinity” is the capability of a dye or pigment to be bound tocellulose fibers. Cellulosic materials are slightly anionic in water dueto partly dissociated carboxylic acid and other functional groups. Somechemically treated pulps may also contain sulfonate groups.

The anionic character of cellulosic materials in water affects thesubstantivity and affinity of dyes and pigments for paper. Thus, anionicdyes such as acid and anionic direct dyes will typically require theaddition of fixing agents to overcome electrostatic repulsion from theanionic cellulose fibers. Cationic dyes such as basic and cationicdirect dyes will be electrostatically attracted to the anionic cellulosefibers, but may still require fixing agents to achieve acceptablesubstantivity and affinity.

Pigments have not enjoyed the field of coloring paper. about 60% of thepaper market, and acid dyes and pigments make up the remainder. SeeMurray, “Dyes and fluorescent Whitening Agents for Paper,” PaperChemistry 161-192 (2d ed. 1996). This lack of market penetration may beexplained by the fact that pigments do not contain solubilizingfunctional groups and have little affinity for or substantivity tocellulose. In particular, the addition of a fixing agent, such ascationic starch, aluminum sulfate (alum) and cationic polymers, istypically required to fix pigments to cellulose fibers.

Aluminum sulfate is the most common fixing agent for pigments and canalso serve as an acidic sizing agent. However, neutral sizing agentshave gained in popularity over acidic sizing agents, and aluminumsulfate can interfere with neutral sizing agents.

An object of the invention is to provide a method for coloringcellulosic materials using an aqueous pigment dispersion which does notrequire fixing agents or alum.

A feature of the method of the present invention is the use of acationic dispersion containing at least one pigment, water, and at leastone dispersing agent comprising a quaternary salt of a styrene maleimideimide resin to color cellulosic materials such as paper.

An advantage of the method of the present invention is that it permitsconsistent coloring of cellulosic material over time, which is importantin continuous and semi-continuous papermaking operations.

Yet another advantage of the method of the present invention is that itexhibits essentially 100 percent, rapid exhaustion of the pigmentparticles into the cellulosic material, and thus generates clearbackwaters. This is vitally important both from an economical andenvironmental vantage point.

SUMMARY OF THE INVENTION

In one aspect, the present invention relates to a method of coloring acellulosic material, which includes

-   -   a) dispersing pulped cellulosic material into water; and b)        coloring the pulped cellulosic material by adding a cationic        dispersion to the water, where the dispersion includes:        -   (i) at least one pigment;        -   (ii) water; and        -   (iii) at least one quaternary salt of a styrene maleimide            imide resin in an amount effective to disperse the pigment.

In another aspect, the present invention relates to a colored cellulosicmaterial, consisting essentially of pigment particles coated with astyrene maleimide imide resin; the coated particles fixed on fibers of acellulosic material.

In yet another aspect, the present invention relates to a cationicdispersion, which includes

-   -   (i) at least one pigment;    -   (ii) at least one dispersing agent comprising a quaternary salt        of a styrene maleimide imide resin; and    -   (iii) water.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Inorganic and organic pigments may be used in the cationic dispersion ofthe present invention. Suitable inorganic pigments include red oxide,yellow oxide, black iron oxide, cobalt blue, carbon black and bismuthvanadate (yellow 184).

Suitable organic pigments may be chosen from azo pigments, such as azolake, azo chelate and condensed azo pigments, and polycyclic pigmentssuch as phthalocyanine pigments, perylene pigments, perinone pigments,anthraquinone pigments, quinacridone pigments, dioxazine pigments,thioindigo pigments, isoindolinone pigments, quinophthalone pigments,rhodamine pigments, arylide pigments, diarylide pigments and naphtholred pigments. Preferred organic pigments include phthalocyanine green,phthalocyanine blue, carbazole violet, toluidine red, perylene red,quinacridone red, quinacridone yellow, quinacridone violet, arylideyellow, Dalamar yellow, Watchung red, and diketopyrrolopyrrole (DPPred).

The cationic dispersion of the present invention comprises at least onequaternary salt of a styrene maleimide imide resin. Styrene maleimideimide resins may be prepared by reacting a styrene maleimide resin witha primary diamine, such as dime thylaminopropylamine, to form a styrenemaleimide imide having tertiary amine functional groups. Further detailson these styrene maleimide imide resins are found in “TechnicalInformation—Styrene Maleimide Resins SMA X 1000 I, X 2000 I, X 3000 I, X4000 I,” Elf Atochem Brochure (1998), the disclosure of which isincorporated by reference herein in its entirety.

It is possible to prepare copolymer resins having a styrene/maleimideratio ranging from 1/1 to 1/4 depending on the base resin employed inthe imidization reaction. A particularly preferred styrene maleimideimide resin is prepared by reacting dimethylaminopropylamine with astyrene maleimide resin, commercially available from ATOFINA Chemicals,Inc., Philadelphia, Pa. (formerly known as Elf Atochem, Inc.).Imidization can be performed using a non-reactive diluent, the desiredamine, and the styrene maleimide resin. Typical reaction conditions are150-180° C. for 30-40 minutes. The generalized reaction scheme is setforth below:

where n may be 1-3 and m is 6-8.

These styrene maleimide imide resins are insoluble in water. However,they may be converted to their corresponding quaternary salts, which arewater soluble, by reaction with a weak acid. The weak acid may have anacid dissociation constant Ka of 1×10⁻² to 1×10⁻⁷. Illustrative weakacids include acetic acid, citric acid, acid, hydrofluoric acid, oxalicacid and nitrous acid, preferred. The pH of the cationic than 7,preferably between 4 and 6.

The cationic dispersion of the present invention typically 10% by weightpigment. The dispersing agent is in an amount of from 1-20%, preferably3-8%, by weight. A slight excess of the weak acid may be present toensure that the quaternary salt of the styrene maleimide imide remainssolubilized. When acetic acid is employed, a ratio of about 1:5 aceticacid/resin is normally sufficient to ensure the solubility of thestyrene maleimide imide resin quaternary salt.

Other additives may be present in any amount which does not detract fromthe cationic dispersion's cellulosic materials such as paper. Suitableadditives include viscosity control agents, surfactants and biocides.Suitable viscosity control agents include hydroxyalkyl celluloses, suchas hydroxyethylcellulose, which are preferably added to the cationicdispersion to increase its viscosity to a 10,000 centipoise, preferably1,000 to 2,000 centipoise, at 25° C. The amount of viscosity controlagent will depend on the relative amounts of dispersing agent, pigmentand water forming the dispersion, and may range from 0.05% to 2% byweight of the dispersion.

One or more surfactants may optionally be added to the dispersion to aidin its manufacture if the surfactant does not create foam. For example,non-ionic surfactants having a hydrophobic/lipophobic balance (HLB) lessthan 13, also known as grind aids, may be added to the dispersion todecrease milling time by reducing the surface tension of thepigment/water interface. An illustrative grind aid is an acetylenic diolwith an HLB of 3 which is commercially available under the trademarkSURFYNOL 104 from Air Products, Inc., Allentown, Pa.

Surfactants may also be added to the cationic dispersion to reducefoaming during mixing of the dispersion. Suitable defoaming agentsinclude mineral oils, silicone polymers and acetylenic diols. Adefoaming agent comprising a mixture of dipropylene glycol andtetramethyl-6-dodecyne-5,8-diol, commercially available from AirProducts, Inc. under the trademark DF110D, is preferred. A concentrationof about 0.1 weight percent is normally sufficient to ensure thedispersion does not foam during mixing. A biocide may also be added tothe cationic dispersion. Suitable biocides include octhilinone,bromonitroalcohol, formaldehyde and formaldehyde-based derivatives. Aconcentration of about 0.1 weight percent is normally sufficient toensure no harmful or objectionable bacteria colonize the dispersion.

The cationic dispersion of the present invention may be prepared by athree-stage process. In the first stage, the pigment, styrene maleimideimide quaternary salt, and water, together with any desired optionaladditives such as a surfactant and/or biocide, are mixed together in thedesired amounts to form a dispersion premix. Conventional high speedmixing equipment may be used without modification. A mixing speed offrom 500 to 10,000 rpm for a time period of from 1 minute to 2 hours,preferably 10-25 minutes, may be used depending on the size of thebatch. One of ordinary skill in the art will readily understand that thedispersion of the present invention can also be prepared using a styrenemaleimide resin rather than its corresponding quaternary salt, if a weakacid is also added to solubilize the styrene maleimide imide resin perse.

In the second stage, the dispersion premix is media milled, typicallyusing ceramic, metal or glass beads, to reduce pigment agglomerates toprimary particles, thereby forming a non standardized dispersion. Mediamilling can be performed using conventional milling equipment withoutmodification.

In the third and final stage, water is added to the nonstandardizeddispersion until the color of the dispersion matches a color standard.Generally from 5 to 10% by weight water is required to standardize thedispersion.

The cationic dispersion of the present invention may be used to colorcellulosic materials such as paper and cotton using conventionaltechniques and apparatus. For example, the cationic dispersion may beadded to conventional paper pulp, such as mechanical pulp or chemicalpulp, as it is being made into paper. Thus, for example, from 0.05% to10% by weight, preferably 2-3% by weight, of the cationic dispersion maybe added to an aqueous solution of paper pulp, and homogenized for atime sufficient to completely exhaust the pigment into the cellulosicfibers of the paper prior to paper sheet formation.

The styrene maleimide imide quaternary salt is only soluble in an acidicsolution, and becomes insoluble in an alkaline environment. Those ofordinary skill in the papermaking arts know that water quality can varytremendously, particularly if river water is used rather than municipalwater. In particular, pH can range from 4 to 9. Accordingly, it may benecessary to monitor and, if necessary, adjust the pH below 7 to ensureoptimum performance of the cationic dispersion.

Without intending to be bound by theory, the inventors currently believethat the cationic styrene maleimide imide quaternary salt coats thepigment particles, thereby allowing them to disperse in water. When thedispersion is mixed with an aqueous solution of anionic cellulosicmaterials such as paper pulp, the cationic styrene maleimide imidequaternary salt is electrostatically attracted to the anionic, partiallydissociated carboxylic groups of the cellulosic fibers, fixing thecoated pigment thereon.

The method of the present invention provides a colored cellulosicmaterial which does not require a fixing agent for the pigment. Yetanother advantage of the essentially complete exhaustion of the pigmentinto the cellulosic material and a correspondingly clear backwater.

EXAMPLES

The following examples illustrate preferred embodiments of theinvention, and are not intended to limit the scope of the invention inany manner whatsoever.

Example 1

Formulation of a Cationic Dispersion Containing Blue Pigment

A high speed mixer was used to mix acetic acid, phthalocyanine bluepigment, styrene maleimide imide resin (SMA x 2000 I, commerciallyavailable from ATOFINA Chemicals, Inc., Philadelphia, Pa.), a defoamingagent comprising a mixture of dipropylene glycol andtetramethyl-6-dodecyne-5,8-diol, commercially available from AirProducts, Inc. under the trademark DF110D, a biocide comprisingocthilinone, commercially available from Thomson Research Associates,Toronto, Canada, under the trademark ULTAFRESH DM-25, and water to forma dispersion premix, which was then media milled (Eiger mixer) todisperse and incorporate the pigment into the dispersion had a totalsolids percentage of percentage of 48.7. The weight percentagecomposition of this cationic dispersion is set forth below in Table 1:TABLE 1 MATERIALS WEIGHT PERCENTAGES Styrene Maleimide Imide 5.50Pigment (Phthalo Blue) 43.00 Weak Acid (Acetic Acid) 1.00 Defoamer 0.10Biocide 0.10 Water 50.30 TOTAL 100%

Example 2 Formulation of a Cationic Dispersion Containing Yellow Pigment

A second cationic dispersion was formulated using the general proceduresof Example 1. The weight percentage composition of the resultingcationic dispersion is set forth below in Table 2: TABLE 2 MATERIALSWEIGHT PERCENTAGES Styrene Maleimide Imide 5.50 Pigment (Phthalo Blue)43.00 Weak Acid (Acetic Acid) 1.00 Defoamer 0.10 Biocide 0.10 Water50.30 TOTAL 100%

Example 3 Coloring of Paper Pulp

The cationic dispersions of Examples 1 and 2 were each individually usedto color paper pulp in accordance with the following procedure: 4 gramsof a 50/50 blend of hard and soft wood fibers were added to a beakercontaining 100 grams of water and mixed for approximately 5 minutesusing a flat mixing blade operating at a speed of at least 100 rpm toproduce an aqueous suspension of cellulosic fibers.

Separately, 1 gram of the cationic dispersion was diluted with 250 gramsof water. 25 milliliters of the diluted dispersion were pipetted intothe aqueous suspension, which was mixed for another 5 minutes using thesame mixing conditions and equipment, thus resulting in an aqueoussuspension of colored cellulosic fibers.

The aqueous suspension was then put in a small sheet mold having aforming screen on the bottom, and the water was extracted, therebyforming a sheet of colored paper on the forming screen. Both of thecationic dispersions completely exhausted their pigments into the paperpulp, and gave crystal clear backwaters. The colored paper was blottedand dried on a small paper drier.

The completely dry colored paper was evaluated for color continuity, twosidedness, color matching to a standard, and color strength. Samples ofcolored paper made from the cationic dispersion of Example 1, andsamples of colored paper made from the cationic dispersion of Example 2,passed all tests.

1. A method of coloring a cellulosic material, comprising a) dispersingpulped cellulosic material into water; and b) coloring said pulpedcellulosic material by adding a cationic dispersion into said water,wherein said dispersion comprises: (i) at least one pigment; (ii) atleast one dispersing agent comprising a quaternary salt of a styrenemaleimide imide resin; and (iii) water.
 2. A cationic dispersion,comprising: (i) at least one pigment; (ii) at least one dispersing agentcomprising a quaternary salt of a styrene maleimide imide resin; and(iii) water.
 3. The cationic dispersion of claim 2, wherein said pigmentis at least one organic pigment selected from the group consisting ofphthalocyanine green, phthalocyanine blue, carbazole violet, toluidinered, Dalamar yellow, Watchung red and diketopyrrolopyrrole, quinacridonered, quinacridone yellow, quinacridone violet and arylide yellow.
 4. Thecationic dispersion of claim 2, wherein said organic pigment is aphthalocyanine.
 5. The cationic dispersion of claim 2, wherein saidpigment is at least one inorganic pigment selected from the groupconsisting of red oxide, yellow oxide, black iron oxide, cobalt blue,carbon black and bismuth vanadate.
 6. The cationic dispersion of claim2, further comprising at least one member of the group consisting of asurfactant, a biocide and a viscosity control agent.
 7. The cationicdispersion of claim 2, wherein said pigment comprises primary particles.8. A colored cellulosic material, consisting essentially of pigmentparticles coated with a styrene maleimide imide resin; said coatedparticles fixed on fibers of a cellulosic material.
 9. The coloredcellulosic material of claim 8, wherein said cellulosic material isselected from at least one member of the group consisting of paper andcotton.